Acetals of 1,2,3,4-tetraoxobutane: preparation and structure determination of rac- and meso-2,3,3,4,4,5-hexamethoxy-tetrahydrofuran Oxidation of 3,4-dimethoxyfuran with 2 equiv. of bromine at −75° in the presence of triethylamine and methanol leads to a mixture of rac- and meso-2,3,3,4,4,5-hexamethoxy-tetrahydrofuran (cf. Scheme 1, 4 and 5). The structure of the crystalline rac-compound has been determined by X-ray analysis.